Add the tool "nixos-typecheck" that can check an option declaration to:

 - Enforce that an option declaration has a "defaultText" if and only if the
   type of the option derives from "package", "packageSet" or "nixpkgsConfig"
   and if a "default" attribute is defined.

 - Enforce that the value of the "example" attribute is wrapped with "literalExample"
   if the type of the option derives from "package", "packageSet" or "nixpkgsConfig".

 - Warn if a "defaultText" is defined in an option declaration if the type of
   the option does not derive from "package", "packageSet" or "nixpkgsConfig".

 - Warn if no "type" is defined in an option declaration.

1 files changed, 3 insertions, 0 deletions

diff --git a/nixos/modules/services/x11/window-managers/xmonad.nix b/nixos/modules/services/x11/window-managers/xmonad.nix
index 6af88d4f645b..d8acb824f7df 100644
--- a/nixos/modules/services/x11/window-managers/xmonad.nix
+++ b/nixos/modules/services/x11/window-managers/xmonad.nix
@@ -16,6 +16,7 @@ in
     services.xserver.windowManager.xmonad = {
       enable = mkEnableOption "xmonad";
       haskellPackages = mkOption {
+        type = lib.types.packageSet;
         default = pkgs.haskellPackages;
         defaultText = "pkgs.haskellPackages";
         example = literalExample "pkgs.haskell.packages.ghc784";
@@ -28,7 +29,9 @@ in
       };
 
       extraPackages = mkOption {
+        type = lib.types.functionTo (lib.types.listOf lib.types.package);
         default = self: [];
+        defaultText = "self: []";
         example = literalExample ''
           haskellPackages: [
             haskellPackages.xmonad-contrib